Heart augmentation system provided with means for measuring intra-arterial pressure



Patented July 29, 1969 I 3,457,909 HEART 'AUGMENTATION SYSTEM PROVIDEDWITH MEANS FOR MEASURING INTRA-AR- TERIAL PRESSURE John D. Laird,Westford, Mass, assignmto Avco Corporation, Cincinnati Ohio acorporation of Delaware Filed July '20, 19 66, Ser. No. 566,659 7 Int.Cl. A6lb 19/00 US. Cl. 128-1 SCIaims ABSTRACT on THE DISCLOSUREApparatus for providing an electrical signal which is representative ofintra-arterial pressure-This 1S accomplished by providing an electricalsignal proportional to the pressure drop between an actuating pump andblood pump and subtracting this signal from an electrical signalproportionalto the driving pressure at the actuating pump.

This invention relates to circulatory assist systems and moreparticularly to apparatus for measuring pressures in circulatory assistsystems.

The advent of open heart surgery has presented to the medical professionthe opportunity of repairing damaged ordiseased hearts of individualsand where appropriate, using circulatory assist systems in individualswho without such correction and/or systems face premature death. Manydevices are involved in this type of surgery. For example, onecirculatory assist system may comprise an auxiliary ventricle orvalveless blood pump connected across the arch of the aorta and drivenby fluid pressure in response to electronic signals (QRS wave) providedby the heart itself. By operating the blood pump or auxiliary ventriclein proper phase,the systolic pressure in the left heart can be reducedand the systemic circulation can be maintained with a substantiallyreduced work load on the heart muscle. In addition, the operation of theauxiliary ventricle has the effect of shifting the phase of the normalsystolic pressure so that this pressure appears in the aorta at a timewhen the left ventricle is relaxed. Assuming competence of the normalaortic valve, one

damaged sections'of an artery. Edwards Seamless Arterial Graftmanufactured by the United States Catheter and Instrument Company havebeen found to be satisfactory.

In most, if not all, circulatory assist systems, it is necessary thattheflexible bulbbe synchronized with the patients heart. A typicalpneumatically driven and electrically controlled circulatory assistsystem is disclosed in US. Patent No. 3,099,260. Other systems aredisclosed in patent application No. 355,273 filed Mar. 27, 1964 nowabandoned, and patent application No. 531,281 filed Mar.

2, 1966, to which reference is made and which areas- I signed to thesame assignee as this application.

In the use of circulatory assist systems, it is desirable if notnecessary to be able to measure intra-arterial pressure or, which isessentially the same thing, the pressure in the auxiliary ventricleduring the time it is in use. This is desirable for medical as well asoperational and maintenance reasons. Accordingly, a feature of thepresent-invention is the, provision of apparatus in a circulatory'assistsystem for measuring intra-arterial pressure. a Another featureof the present invention is the provision of apparatus in a circulatoryassist system which provides an electrical signal representative ofintra-arterial pressure or the pressure in an auxiliary ventricle.

A further feature of the present invention is the measurement of a partof the pressure drop along a tube connected between an actuating unitand an auxiliary venthen has an increased perfusion pressure availableto the I coronary arteries. It is believed that such an increase incoronary perfusion, together with a reduction in the effort requiredfrom the heart, should be effective in a number of cases of cardiacinsufficiency.

As'may be seen from the above, one important component of circulatoryassist systems is a pump that either assumes the hearts role .of pumpingblood or which reduces the work load of the heart muscle. By using heartpump equipment for extended periods of time, it is contemplated that theequipment may be utilized for regional perfusions in therapeutictreatment of the heart. Still other use of the equipment will be toprovide circulation of blood through an artificial organ such as anexternal artificial kidney. In connection with this function of theapparatus, it should be noted that many research institutions at thistime are concentrating their research activities on providing artificialcounterparts of other organs, and whenever such application requirescirculation,

the Present invention may be utilized.

Implantable prior art pulsatile pumps usually consist of a flexible bulbor ventricle squeezed by pressurized fluids fromIa pumping or actuatingunit and is coupled to one or more blood vessels such as arteriesor.veins.

Generally, arterial graft sections connect the bulb to the graftsections are gen- 7 tricle which is converted to an electrical signalproportional to the total pressure drop along the tube and subtractedfrom an electrical signal proportional to the driving pressure at theactuating unit to provide an accurate representation of the pressure inthe auxiliary ventricle.

A still further feature of the present invention is the provision ofapparatus in a circulatory assist system which provides an electricalsignal which is accurately representative of the pressure in theauxiliary ventricle.

A still further feature of the present invention is the provision in acirculatory assist system of apparatus for 'measuring intra-arterialpressures which permits a visual display accurately representative ofthe work done by the pumping unit.

The novel features that are considered characteristic of the presentinvention are set forth in the appended claims; the invention itself,however, both as to its organization and method of operation togetherwith additional objects and advantages thereof, will best be understoodfrom the description of a specific embodiment when read in conjunctionwith the accompanying drawing which is a schematic illustration of atypical circulatory assist system incorporating apparatus in accordancewith the present invention.

Directing attention now to the drawing, there is shown a schematicillustration of heart pumping or circulatory assist apparatus intendedto provide intracorporeal mechanical assistance and incorporating thepresent invention. As shown in the drawing, in a typical system asuitable pressurized source of gas 11 feeds into a low pressureregulator 12. Large oxygen bottles which are readily available and are asatisfactory source of oxygen are generally pressurized to a pressure ofseveral thousand pounds and generally have a pressure regulator which,while not particularly sensitive, is satisfactory to provide a reductionin pressure approaching that required for the actuation of the bloodpumping unit. A satisfactory pressure for the pumping unit has beenfound to be approximately 3 pounds per square inch; hence, pressureregulator 12, while of conventional design, should permit smalladjustmentsjn thc pressure range of. about 10 to 3 pounds per squareinch. The output of the low pressure regulator o 12 is fed toa three-waysolenoid actuated valve 13. The

valve 13, which is normally open to the atmosphere, is adapted to beoperatcd by a synchronizing circuit 14 and allows compressed gas to besupplied to an actuating unit 15. Thus, only when the valve 13 isactuated by the synchronizing circuit 14 does the valve 13 supplycompressed gas to the actuating unit 15 which in turn controls theaction of the pumping unit 16.

Broadly, the action of both the actuating unit and the pumping unit mustbe capable of being synchronized with the patients heart. The actuatingunit and, hence, the pumping unit must be capable of being phased withthe patients heart while the duration of the systole and diastolestrokes should be adjustable. The synchronizing circuit 14 performs thefunction of properly synchronizing the operation of the solenoid invalve 13 for admitting the pressurized gas into the actuating unit 15 inaccordance with the demands of the patient. Typically, the synchronizingcircuit is actuated by the patients electrocardiogram or the R-wavetaken directly from his heart. By way of example, the output of an EKGunit may be fed into an amplifier and synchronizer pulse circuit that isadapted to amplify the sync pulse or electrical signal used forsynchronizing purposes. The amplifier and synchronizing pulse shapcr ifprovided may be designed not only to limit the magnitude of the syncpulse but also to shape it. Since the actuating unit is designed to besynchronized with the R-wave of the sync pulse, all other portions ofthe wave may be either reduced or removed, thereby leaving only theR-wave. Since the hydraulic events in the patients heart are notsimultaneous with the EKG unit or the R-wave and, furthermore, since thehydraulic events in the patient's circulartory system are delayed behindthe systolic pulse of the heart by varying amounts depending on thedistance of the artery or vein from the left ventricle of the heart, itis desirable to provide means for phasing the systolic pulse of thepumping unit with the systolic pulse of the heart in order toaccommodate these time delays and provide the desired time delay. Forthis purpose, a systole delay network triggered by the R-wave may beprovided to create a sync pulse delayed behind the R-wave by acontrolled amount to enable the systolic pulse of the pumping unit to bedelayed behind the systolic pulse of the patient's heart by anappropriate time interval. By providing this time delay interval, thepumping unit may be adjusted so that the pressure reflections from thesystolic pulse of the pump ing unit will be properly phased with thepressure reflections from the systolic pulse of the patients heart andin such a way as to physiologically aid the patients heart.

The synch pulse produced by the aforementioned systolic.delay networkmay be utilized to actuate a trigger circuit which may include a systoleduration control circuit which is provided for controlling the durationof the tripped condition of the trigger circuit. The output of thetrigger circuit may be fed directly into an amplifier, the function ofwhich is to create a signal for firing a thyratron switching circuit orthe like, which controls the operation of the three-way solenoid valve13. For a further discussion of suitable synchronizing circuits fordifferent applications, reference is made to the aforementioned U.S.Patent No. 3,099,260, and patent application Ser. No. 355,273.

Directing attention now to the actuating unit 15, it may be of the typedisclosed in the aforementioned patent but is preferably of the typecomprising a low inertial diaphragm separating the unit into an inputcompartment and an output compartment, the pressurized gas from valve 13being admitted into the input compartment and the gas in the outputcompartment being in communication with the pumping unit 16 through asecond threeway solenoid valve 24 and a percutaneous connector 25.

The actuating unit preferably is provided with stops to prevent thediaphragm from providing a volumetric displacement substantially greaterthan about 60 cc. which is in the range of the average volumetricdisplacement of the left ventricle of the human heart. Further, theactuating unit should have a low resistance to maintain the load on theheart as low as possible since the heart must move the diaphragm unlessthe input compartment is coupled at the appropriate time to a partialvacuum through valve 13 during diastole.

Mounted or affixed to the actuating unit is a transducer 27 actuated bythe movement of the diaphragm in'the actuating unit 15. This may beaccomplished in conventional fashion for example by providing amechanical connection such as a rod between the transducer 27 and theaforementioned diaphragm in the actuating unit 15. While the particulartype of transducer used is not critical, it should preferably provide adirect current signal, the magnitude of which is proportional to themovement of the diaphragm. Thus, if the diaphragm is moving, the' outputsignal of the transducer. 27 will be a varying signal and if thediaphragm stops in any particular place, the output signal will be adirect current voltage.

The actuating unit 15 is coupled to the pumping unit 16 through asolenoid actuated valve 24. The output signal of the transducer 27 isfed to fail-safe circuitry 28 which controls the solenoid valve 24. Thesolenoid valve 24 is normally open, i.e., if the diaphragm isinoperative, the environment surrounding the collapsible bulb of thepumping unit is vented to the atmosphere. A typical pumping unitcomprises a rigid container containing a collapsible bulb, the outersurface of which is in communication with a pressurized gas (the outputcompartment'of the pumping unit 15) and the inner surface of which is incommunication with the circulatory system of the body. A typicalextracorporeal ventricle is disclosed in the aforementioned U.S. PatentNo. 3,099,260, and a typical intercorporeal ventricle is disclosed inthe aforementioned patent application Ser. No. 355,273.

All of the foregoing components with, of course, the exception of thepercutancous connector and pumping unit may be located in a bedsidecontrol panel. Tube 31 connects the actuating unit 15 to the valve 24,both of which are in the control panel. Tube 33 connects the pneumaticportion of the system to the patient in which is implanted thepercutaneous connector 25 and the pumping unit 16. Tube 32 which isdisposed interior of the body connects the percutaneous connector to thepumping unit.

Having now described a typical circulatory assist system, attention isdirected to the apparatus in accordance with the invention comprisingdrive pressure transducer 34, differential pressure transducer 35,amplifier 36, variable gain amplifier 37 and summing amplifier 38. Thedrive pressure transducer 34, which may comprise a typical strain gaugepressure transducer providing an output voltage proportional to pressureis pneumatically coupled to tube 31 at the outlet of the actuating unit.The electrical output signal of the drive pressure transducer 34 issupplied to the input of a conventional linear amplifier 36 having zerophase shift from DC to about 50 cycles per second and the output of theamplifier 36 is coupled to one of two input terminals of a conventionalsumming amplifier 38 which will provide an output signal proportional toeither the difference or sum of the signals supplied to its inputterminals. In the present case, as will be more fully described, thepolarity of the input signals to the summing amplifier should be suchthat the output difference of the input signals.

A conventional differential pressure transducer 35 may be connectedacross a length of the pressure line suflicient to provide a measurablepressure difference or drop in the line as shown by way of example inthe drawing. As will be pointed out hcrcinbelow, the distance betweenthe pressure connections of transducer 35 is not important. Differentialpressure transducer 35 may be any conventional type which provides anelectrical output signal proportional to the difference in pressures itsees through its pneumatic ports. The output signal of transducer 35 iscoupled to an amplifier 37 identical to amplifier 36 except that itsgain must be linearly variable over an appreciable range. The output ofamplifier 37 is coupled to the remaining input terminal of the summingamplifier 38 wherein it is subtracted from the output signal ofamplifier 36.

The flow of gases in a circulatory assist system described hereinabovemay be characterized as an unsteady viscous flow. Such a flow gives riseto two effects which can influence the distribution of pressures alongthe pressure line which connects the actuating unit to the pumping unit.The first of these effects is simple, steady viscous pressure dropcaused by the flow of gas through a relatively small caliber pipe. Thesecond eifect which in part causes the pressure difierence between thepressure measured at the actuating unit and the pressure actuallyexisting in the pumping unit is due to the unsteady nature of the flow.This second effect may be compared to the socalled water hammer efrecand is the result of the rapidly accelerating flow in the early statesof actuation of the actuating unit. Tests performed in the developmentof the present invention have shown that a display of uncorrectedpressure (the pressure at the actuating unit) is not linearlyproportional to the actual pressure within the pumping unit. It is inerror by an amount which is proportional to the square of the velocityof the gas in the pressure line and also a term corresponding to thetime rate of change of the velocity. Both of these effects whilenonlinear functions of time are linearly proportional to the length oftubing. Thus, the losses in the pressure line are implicit functions oftime and give rise to a varying correction throughout the total pressuretime history. It will now be clear that drive transducer 34 cannotprovide a signal accurately representative of the pressure in thepumping unit.

In accordance with the present invention, an electrical signalaccurately representative of the pressure in the pumping unit isprovided by measuring the pressure drop along a small fraction of thetotal line from the actuating unit to the pumping unit with difierentialpressure transducer 35 whose signal is amplified by the variable gainlinear amplifier 37. The gain amplifier 37 is adjustable in proportionto the length of pipe between the actuating unit and the pumping unit sothat the output signal of this amplifier is proportional to the totalpressure drop over the whole line from the actuating unit to the pumpingunit. Pressure transducer 34 just downstream of the actuating unitmeasures the absolute pressure level in the line at the drive end and itis this pressure which must be corrected in order to infer the pressurein the pumping unit. The correct pressure in the pumping unit isinferred in accordance with the present invention by taking the adjustedoutput from variable amplifier 37 which is proportional to the totalpressure drop in the line and subtracting it from the output ofamplifier 36 (which is merely the amplified output of the drive pressuretransducer 34) in summing amplifier 38 to provide a final output voltagefrom summing amplifier 38 that is proportional to the actual pressureexisting in the pumping unit.

The system is calibrated principally by determining the properadjustment for variable gain amplifier 37. The proper adjustment ofamplifier 37, which is to say the adjustment of the gain of amplifier37, may easily be determined in the following manner: The distal end oftube 32 which is normally connected to the pumping unit is connected toa large volume having a pressure comparable to that which would exist inthe pumping unit, such as, for example, 70 to 100 millimeters pressure.It is essential that this calibration volume be large as compared to thestroke volume of the actuating unit. Since the pressure in the largecalibration volume will not change significantly for significantpressure changes at the actuating unit, amplifier 37 can be adjusted toprovide a substantially constant output signal from the summingamplifier. Accordingly, in the calibration procedure, the output of thesumming amplifier may be connected to a scope and the variable gainlinear amplifier adjusted during operation of the actuating unit toprovide as closely as possible 'a constant output voltage from thesumming amplifier. When this is achieved, the output voltage from thevariable gain linear amplifier is, as has been verified by tests,accurately representative of the total pressure drop in the lineconnecting the actuating unit and the pumping unit.

In actual practice, that is, at an implantation of the pumping unit, theamount of line from the actuating unit to the connection on the controlpanel is known and will remain fixed. However, the amount of linebetween the percutaneous connector and the pumping unit may and probablywill vary within certain limits. Therefore, the system may be calibratedat the time the pumping unit is implanted by substituting for the linewhich will be permanently connected to the control panel and thence tothe pumping unit a second line of equal length. This line may then beconnected to the aforementioned large calibration volume having theappropriate pressure as described previously and the variable gainlinear amplifier adjusted for a substantially constant output voltage.At this time, the system will be calibrated and thereafter the permanentline can be used. Subsequent to implantation, the output signal ofsumming amplifier may, for example, be displayed on an oscilloscope topermit observation of the time history of the patients intra-arterialpressure. Operation and/ or maintenance of the entire system can bemonitored by supplying the output signal to an oscilloscope incombination with the output signal of transducer 27 to provide a workdiagram of the pumping unit.

While it is theoretically possible to measure the entire pressure dropfrom the actuating unit 15 to the pumping unit 16, it will beappreciated that the present invention is vastly superior in that itcompletely eliminates the necessity of a second pressure tube which mustbe attached to the patient and the complications and difficulties raisedthereby.

Having now described a preferred embodiment of the present invention,what is claimed is:

1. In a system for assisting blood flow within a living body comprisingmeans for sequentially actuating a blood pumping unit including anactuating unit including a movable pressure member for providingpulsatile pressure through a tube to a blood pumping unit including aflexible bulb in communication with the blood, the combinationcomprising:

(a) first means for providing a first electrical signal proportional tothe pressure at said actuating unit;

(b) second means for providing a second electrical signal proportionalto the pressure drop between said actuating unit and said blood pumpingunit; and

(c) third means for combining said first and second electrical signalsand providing a third electrical signal proportional to the pressure atsaid blood pumping unit.

2. The combination as defined in claim 1 wherein:

(a) said first means includes pressure transducer means for measuringthe pressure at said actuating unit and first amplifier means coupled tothe output of said pressure transducer means for providing said firstsignal; and

(b) said second means includes differential pressure transducer meansfor measuring a portion of the pressure drop between said actuating unitand said blood pumping unit and variable gain amplifier means coupled tothe output of said differential pressure for providing said secondelectrical signal.

3. The combination as defined in claim 2 wherein said third meansincludes summing amplifier means for subtracting said second electricalsignal from said first electrical signal to provide said thirdelectrical signal.

4. The combination as defined in claim 3 wherein each said amplifiermeans is linear and has a substantially Zero phase shift from DC to atleast fifty cycles per second.

5. In a system for assisting blood flow within a living body comprisingmeans for sequentially actuating a blood pumping unit including anactuating unit including a movable pressure member for providingpulsatile pressure through a tube to a blood pumping unit including aflexible bulb in communication with the blood, the combinationcomprising:

(a) first means for providing a first electrical signal proportional tothe pressure at said actuating unit, said first means including pressuretransducer means controlled by the pressure at said actuating unit andproviding an electrical signal proportional to said pressure, said firstmeans additionally including linear amplifier means having substantiallyzero phase shift for receiving said electrical signal from said pres- 15sure transducer means and providing said first electrical signal;

(b) second means for providing a second electrical signal proportionalto the total pressure drop between said actuating unit and said pumpingunit, said second means including differential pressure transducer meanscontrolled by the pressure drop over a portion of said tube andproviding an electrical signal proportional to said portion of saidtotal pressure drop,

said second means additionally including variable gain linear amplifiermeans having substantially zero phase shift for receiving said signalfrom said differential transducer means and providing said secondelectrical signal; and

(0) third means for subtracting said second electrical signal from saidfirst electrical signal and providing an electrical output signalproportional to the pressure in said pump unit.

References Cited UNITED STATES PATENTS 7/1963 Birtwell 128-1 8/1966Watkins et al l28-1 OTHER REFERENCES Hiller et al., Amer. Jour. of Med.Electronics, July- September 1963, pp. 212-221.

DALTON L. TRULUCK, Primary Examiner US. Cl. X.R.

